In many settings in Africa, women with unknown HIV status present late in pregnancy or in labor, and the postpartum period represents a major entry point for counseling, testing, and assessment for antiretroviral (ARV) treatment. ARV use in pregnant and breastfeeding women has clear benefits for mothers and infants. Furthermore, if an infant is already HIV-infected, breastfeeding is seen as posing no risk to the infant, and women are strongly encouraged to continue breastfeeding to maximize survival of the infant. However, use of highly active antiretroviral therapy (HAART) in breastfeeding women may induce ARV resistance in some HIV-infected infants, either through transfer of ARV-resistant strains from the mother to the infant, or by exposing the infant to biologically significant but sub-therapeutic amounts of ARV drugs, leading to selection of ARV-resistant HIV in infants who are already infected with ARV-susceptible HIV. Development of ARV resistance to more than one class of ARV drugs (multi-class resistance) in HIV-infected infants is likely to significantly reduce their chance of responding to life-saving ARV therapy. The hypothesis of this study is that maternal HAART induces multi-class ARV resistance in the majority of HIV-infected breastfeeding infants. This hypothesis will be tested using samples and data from the Post-Exposure Prophylaxis of Infants (PEPI) trial that was conducted in Blantyre, Malawi. In PEPI, approximately 90 women who initiated HAART postpartum had infants who were HIV-infected. Women and infants in PEPI received ARV drugs for prevention of mother-to-child transmission of HIV (pMTCT). Most women (~70%) received single dose nevirapine (sdNVP). All infants received sdNVP and 1 week of zidovudine (control regimen). Infants were then randomized at birth to receive either the control regimen alone, or the control regimen plus up to 14 weeks of extended daily NVP or extended daily NVP plus extended daily ZDV. These regimens were stopped immediately in infants with confirmed HIV infection. Because these pMTCT regimens are known to cause selection of NVP-resistant HIV in some women and some HIV-infected infants, exposure to pMTCT regimens will be considered in our analysis. The Specific Aims of this proposal are: Aim 1: Test whether initiation of maternal HAART postpartum is associated with emergence of multi-class resistance in HIV- infected breastfeeding infants. Aim 2: Analyze HIV viral load, ARV drug levels, and HIV drug resistance in breast milk samples from women on HAART. These studies will quantify the prevalence of multi-class resistance in infant plasma and breast milk after maternal HAART initiation, and will identify factors associated with development of multi-class resistance in HIV-infected breastfeeding infants. These studies will also assess whether infants in this setting are more likely to acquire multi-class resistance from transmission of resistant HIV from the mother (selected from her HAART regimen), or from exposure to ARV drugs in breast milk. This information will facilitate the design and implementation of strategies for HIV prevention and treatment in resource-constrained settings.